The role of zero-tillage in mitigating climate change in tropical soils

Soils are a significant store of organic carbon, globally storing an estimated 1550 Gt C to a depth of 1 metre. They are also substantial sources of greenhouse gas (GHG) emissions, contributing one-fifth of global CO₂ emissions, one-third of CH₄ emissions and two-thirds of N₂O emissions. Soil carbon in agricultural lands can represent a net sink or source of CO₂ depending on microclimate, cropping history and land management. Zero-tillage is an increasingly popular strategy to minimise soil erosion, increase biological activity and promote soil health. However, the extent to which zero-tillage reduces GHG emissions whilst increasing soil carbon, compared to other management strategies, is extensively debated, and represents a crucial knowledge gap in the context of climate change mitigation. Contrasting tillage strategies not only affect the stability and formation of soil aggregates but also modify the concentration and thermostability of soil organic matter (SOC) associated within them. Understanding the thermostability and carbon retention ability of aggregates under different tillage systems is essential to ascertain potential terrestrial carbon storage and greenhouse gas release.

 

Across Brazil, zero-tillage accounts for c. 45% of agricultural management, thereby making it a critical agricultural management practice throughout South America. This has been a popular management strategy since the 1940s and provides long-term field sites for which to understand and elucidate the key mechanisms which govern carbon retention/mineralization across different tillage managements. We measured GHG release and characterized the concentration and thermostability of SOC within various aggregate size classes under both zero and conventional tillage using Rock-Eval pyrolysis. The geometry of the pore systems was quantified by X-ray Computed Tomography and used to link soil structural characteristics to organic carbon preservation, thermostability and GHG release. Soil samples were collected from experimental fields across Brazil, which had been under zero-tillage for as little as one year up to 31 years, and from adjacent fields under conventional tillage.

 

Soils under zero-tillage had significantly increased pore connectivity whilst simultaneously decreasing interaggregate porosity, providing a potential physical mechanism for protection of SOC in the 0–20-cm soil layer. Changes in the soil physical characteristics associated with the adoption of zero-tillage resulted in improved aggregate formation compared to conventionally tilled soils, especially when implemented for at least 15 years. In addition, we identified a chemical change in composition of organic carbon to a more recalcitrant fraction following conversion to zero-tillage, suggesting aggregates were accumulating rather than mineralizing SOC. This study also revealed that, when combining all three GHG fluxes, potential global warming potential from zero-tilled soils was 50% smaller than that of conventionally tilled soils. These data reveal profound effects of different tillage systems upon soil structural modification, with important implications for the potential of zero-tillage to simultaneously increase carbon sequestration and decrease GHG release compared to conventional tillage, contributing to mitigating against climate change in these soils.